Introduction 1.1 Initial Remarks

Let us start with a bit of history. Today, it is hard to imagine how difficult it was to develop basic concepts and ideas of chemistry in the second half of the 19th century. The story about Kekulé’s fight for his benzene structure shows that not all the arguments he used in its favor are valid today [1]. His idea could not be supported by the poor experimental instrumentation of that time. There was no X-ray analysis, no modern spectroscopic techniques and no calorimetry. The idea of the constitution of molecules, that is building them from a certain number of different types of atoms, was established, as well as several experimental findings which demanded rationalization. Among them were optical activity and the existence of a number of different molecules with the same constitution. Pasteur foresaw that the former phenomenon could be related to the positioning of atoms in space but only the van’t Hoff [2] and Le Bel [3] hypotheses on the tetrahedral arrangement of substituents on the tetravalent carbon atom explained most observations known at that time. Interestingly, the independently proposed models differed slightly: that of van’t Hoff was based on a regular tetrahedron, while in the second one used an irregular tetrahedron to represent the carbon atom. This difference was not significant but, remarkably, the more idealized van’t Hoff approach was generally accepted. An illustration from the 1908 German edition of van’t Hoff’s book, showing two stereoisomers of the tetrasubstituted ethane molecule CR1CR2rCR3CR4r shows the way in which molecules were depicted at that time (Figure 1.1). The van’t Hoff and Le Bel hypothesis was met with strong criticism, not always expressed in impartial scientific language. The renowned chemist and editor of the German Journal für praktische Chemie, Prof. Adolf Kolbe wrote: ‘A Dr. H. van ’t Hoff of the Veterinary School at Utrecht has no liking, apparently, for exact chemical investigation. He has considered it more comfortable to mount Pegasus (apparently borrowed from the Veterinary School) and to proclaim in his ‘La chimie dans l’éspace’ how the atoms appear to him to be arranged in space, when he is on the chemical Mt. Parnassus which he has reached by bold fly’.